Electroconductive polymers are widely used to conductivize various substrates. The best known use of this type is in the manufacture of paper for electrostatographic reproductions. In this, the most common application for electroconductive polymers, the polymer is utilized in a conductive coating formulation which is applied to a paper substrate and subsequently overcoated with a photoconductor layer such as zinc oxide. The product paper subsequently is employed reproduction via an electrostatographic type process.
Prior to the present invention, it has been known to make electroconductive paper through the use of certain conductive cationic polymers such as homopolymers of dimethyl diallyl ammonium chloride (see Boothe et al., U.S. Pat. No. 3,544,318) and polymers of other quaternized ammonium compounds such as described in Schaper et al., U.S. Pat. No. 3,486,932.
It is widely recognized that the most important criteria for selecting a conductivizing agent for use in manufacturing conductivized paper or other conductivized substrates include (1) conductivity; (2) filmability; (3) holdout to solvents employed in the manufacturing and use processes of the conductivized substrate (toluene and kerosene in the case of electrostatographic papers); and (4) low tack. Those skilled in the art of employing these conductive agents in the manufacture of conductivized substrates, such as conductive papers, are aware of the need and desirability for improvement in these properties, i.e. the need for increased conductivity and solvent holdout; the need for improved filmability; and/or the need for decreased tack.
It has been recognized also that the electroconductive paper industry advantageously could employ a water insoluble conductive polymer; it being known, of course, that the substantially linear polyelectrolytes now commonly employed in electroconductive paper manufacture, poly-(dimethyl diallyl ammonium chloride) and poly-(vinyl benzyl trimethyl ammonium chloride) for example, are highly water soluble. The water solubility of the electroconductive layer makes it necessary, in present manufacturing procedures, to apply the photoconductive coating formulation (usually zinc oxide together with binders, etc.) from an organic solvent based slurry. It would be desirable, of course, to apply the photoconductive coating from an aqueous-based slurry so as to minimize the need for expensive solvent recovery units and to more easily comply with current and proposed environmental pollution control standards. Use of a water insoluble conductive polymer would permit conversion, at least in part to aqueous systems.
It is well-known that such water soluble linear polyelectrolytes may be made less soluble, or even water insoluble, via covalent cross-linking. However, as the degree of cross-linking is increased, the polymer solution becomes unworkable and gel-like. Thus the cross-linked polymer becomes unworkable before the desired property of decreased solubility is achieved. As used herein, the term, "covalent cross-linking", refers to cross-linking of polymer chains to form a 3-dimensional network via a general mechanism wherein the cross-link is comprised of covalent bonds. Examples of this type of cross-linking are (1) a polymerization reaction in which a multifunctional monomer is employed which is capable of entering into the polymerization reaction to become a part of more than one polymer chain and/or (2) a relatively simple difunctional (or multifunctional) composition which is capable of reacting with pendant functional groups carried on polymer chains to form a covalently bonded bridge. In either case, when the concentration of the cross-linking agent exceeds about 0.5 percent, it may be assumed that the polymer solution will contain substantial gel particles and, at higher concentrations, will become an unworkable, essentially continuous gel. Often, such polymers actually become continuous and unworkable gels at degrees of cross-linking even less than 0.5 percent.
As is known to those skilled in the arts of the manufacture of polymers and of electrographic paper, attempts to develop cross-linkable, and therefore water insolulizable conductive polymers according to the afore described technology have been largely unsuccessful. The extremely low degree of cross-linking necessary to convert the polymer to unworkable form has lead to significant problems such as (1) very limited shelf life; (2) products which are susceptable to premature cross-linking which can cause severe inefficiencies and waste in the manufacture of the conductivized paper.
An alternate approach to the formation of insolublizable conductive polymers is via the mechanism of ionic bonding. Schaper et al., U.S. Pat. No. 3,579,613, and Michaels, Ind. Eng. Chem., Vol. 57, No. 10, pp. 32 ff, October 1965, have investigated one aspect of this approach. These researchers prepared the polysalt complexes, both stoichiometric and non-stoichimetric, of a polymer containing strongly anionic pendant groups (typically sulfonated acrylates and sulfonated polystyrene) and a polymer containing strongly cationic pendant groups (typically polydimethyl diallyl ammonium chloride and poly-vinyl benzyl trimethyl ammonium chloride). As noted by Michaels, similar polysalt complexes prepared from weakly acidic polyanions and weakly basic polycations yielded gel-like or quasi-liquid coacervates of indefinite chemical composition and high water content with little demonstrable utility. The properties of the polysalt complexes formed from strong acids and strong bases, however, were extensively investigated.
Results of the Michaels work showed that the polysalt complex formed from the interaction of strongly acidic polyanions and strongly basic polycations precipitates from (i.e. is insoluble in) common solvents and exhibits surprisingly high d.c. resistivity. These otherwise intractable polysalt complexes were found to be soluble in selected ternary solvent mixtures comprised of water, a water miscible organic solvent such as acetone and a strongly ionized simple electrolyte such as sodium bromide. The solubilized polysalt was a transparent, homogeneous, viscous sirup.
While polysalt complexes of the type disclosed by Michaels (those formed from the interaction of the polymer of a strongly acidic moiety and the polymer of a strongly basic moiety) show utility in several areas, the most prevalent uses employ the polysalt complex as a solid (note Michaels, U.S. Pat. No. 3,271,496 and Michaels et al., U.S. Pat. No. 3,276,598). For example, the polysalt complex when doped with concentrated electrolyte can be dispersed as a fine powder in plastics to impart antistatic properties or it can be formed as a film and utilized as battery separators, fuel cell membranes, or dialysis membranes and the like.
Further, polysalt complexes of the strong acid-strong base type, after special treatment, have been applied by Michaels to certain substrates and dried to yield a transparent, conductive coating. Although these polysalt complexes, when free of extraneous electrolyte, showed high d.c. resistivity, it was found that when the complexes were equilibrated with highly concentrated electrolyte solutions they become effective d.c. conductors. The use of such electrolyte doped polysalt comples solutions has been proposed in the manufacture of conductive coatings (note also Dolinsky and Dean, Chem. Tech., pp. 304 ff, May 1971).
The Michaels polysalt complex, however, suffers major deficiencies in application to electroconductive coating manufacture. Its high d.c. resistivity absent doping; its lack of solubility in common solvents and the very limited conditions under which it may be solubilized mitigate against its usefulness in many electroconductive coating applications. The fact that such polysalt complexes are soluble only under very selected conditions limits their utility in any area which requires the use of polymer solutions. For example, such polysalt complexes could not be used readily in the manufacture of electroconductive papers by present standard procedures which involve formulation of the conductive polymer into an aqueous slurry of a pigment such as clay, calcium carbonate, etc., and binder system such as hydroxy ethylated starch, polyvinyl alcohol, various synthetic latices, etc. The strong acid-strong base polysalt complex could not be employed in such procedure since (1) variation in the aqueous part of the ternary solvent system encountered in formulation would cause precipitation of the polysalt; (2) the electrolyte doping and the electrolyte used in the ternary solvent system for the polysalt is an undesired component in the coating formulation; and (3) the use of the required water-miscible organic solvent would require resort to solvent recovery units in order to meet environmental pollution control standards.
More recently, liquid polysalt complexes have been prepared from mixtures of anionic polyelectrolytes and cationic polyelectrolytes wherein at least one of the polyelectrolytes is weak (see Economou, U.S. Pat. No. 3,660,338). These polysalt complexes have found utility as dry strength agents in paper manufacturing when employed in combination with a water soluble ionization suppressor designed to prevent coacervation of the polysalt in water in concentrations of between 1 and 10 percent.
Thus, up to the present, polysalt complexes have been prepared as suspensions or sols which, with coacervation, may form gels, or three-dimensional water and hydrocarbon liquid insoluble gel-like structures.